This is interesting. I would hypothesize that the level of oil production of your skin can be slightly altered by changing the pH of your skin. The sebum produced by skin would have it's own pH level and might be the only way your skin could change the surface pH.Should the pH be off, sebum production may be altered (either less or more produced) in an attempt to bring the skin's pH level back into what your body's desired level is.

More info on pH, compiled by tangal (former poster at the Diana Yvonne Skin Care Board) and edited by me (this is a work in progress, and I am still checking some facts. If you spot any problems, don't hesitate to PM me).

The initials pH stand for â€śPower of Hydrogenâ€ť or "Potential of Hydrogenâ€ť. Both terms are correct. This is a measure of the hydrogen ion content in a solution.

The pH scale measures how acidic or alkaline a product is, compared to pure distilled water (which is pH 7.0). Only wet substances have a measurable pH. The skin's pH is actually the pH on the skins surface from the moisture within the skin, and the "acid mantle" which is part of the protective "barrier" on its surface. Hair for example has no pH.

The acid mantle is a fine film layer, slightly acidic, made up from skin oils, sweat, and dead cells. It is one means the skin uses to protect itself from bacteria, moisture loss, and environmental damage. Pollutants, harmful bacterium, contaminants are normally alkaline in nature, so a slightly acid skin surface helps fight off these harmful elements and prevent them from entering into, and damaging the skin. Maintaining the acid mantle is recommended for good skin health. When the acid mantle is damaged it takes approximately 14-17 hours to repair itself (assuming no other damaging products are applied to it).

A damaged acid mantle leads to a number of skin issues, such as over dry skin, dehydration, over oily skin, flaky skin, acne, sensitivity, etc. It would require another long post to detail all this. But incorrect pH products can contribute to acid mantle degradation, and increases in acne-causing bacteria on the skin. (More on this below)

The pH Scale

The pH scale goes from 0-14. This covers most wet substances, and certainly all skincare products.

-Acids have pH values under 7 - they are more acidic then water (acid)-Alkalis have pH values over 7 - they are more alkaline then water (base)-If a substance has a pH value of 7 - it is neutral, like water (neither acid nor base)

The difference between each whole-value pH level represents a tenfold change. (For example, a cleanser with a pH of 6 is ten times more alkaline than a cleanser with a pH of 5, and a cleanser with a pH of 7 is 100 times more alkaline then one of 5.) Because of this, only a few units of pH can make a big difference in how your skin reacts to a product.

Newborn baby skin has a neutral pH of 7. Within a few months it adjusts to a more "normal" pH of 4.5 to 5.5, enabling it to be more resistant to bacteria.

Adult skin is normally slightly acidic, a range of 4.5 to 5.5. Different body areas can differ in pH, and disease and acid mantle damage can alter pH. But the preferred range for facial skin pH is 4.5 to 5.5.

Those with skin disease, skin problems, and stressed skin usually have a skin pH over 6.0. Aged, stressed and damaged skin have more difficulty maintaining a â€ścorrectâ€ť pH.

As skin pH is elevated closer to pH 7.0, it becomes less and less able to function optimally and to kill bacteria. This allows acne-causing bacteria to multiply rapidly on the skin's surface. (Acne bacteria are found on everyoneâ€™s skin, even if you never have a pimple in your life.) The damaged skin is unable to fend off the excess bacteria, and they multiply rapidly. Bacteria growth is very slow at pH of 5.5 or less, but a slight shift upward, toward the alkaline levels, causes a marked increase in the reproduction and lifespan of acne-causing bacteria.

So if your skin is at a higher pH (anything over 6, as damaged skin often is) the acne-causing bacteria can multiply much easier and faster -- often faster then your skin can handle. Skin pH is one main contributor to acne.

Skin pH also has an effect on how easily irritated your skin is, how well it ages, and how it deals with product and environmental stress. This is why it is recommended that you cleanse skin with mild, non-irritating products, as close to the skin's natural pH as possible.

Many commercial cleansers are highly alkaline, which also changes the skin's pH to alkaline levels on the skin surface, for a short time. Many alkaline cleansers are in the same pH range as baking soda, and some are nearly as high as ammonia. This is very harsh on the skin, and can lead to increased irritation, acne, moisture loss, skin aging etc. As pimples erupt, the skin is less able to heal itself, or the damage that pimples leave behind.

If a product has a high pH and a considerable percentage of a strong detergent such as sodium lauryl sulfate, or irritant like peppermint oil or menthol, because of the pH destructive activity on the acid mantle, the detergent can contribute to even more damage then it would if the product pH was closer to 5.5. It literally takes a split second for an alkaline product to degrade the skin barrier enough for an irritant or damaging detergent to penetrate. Some people can handle this better then others, but long term daily use on the skin can contribute to long term issues on all skin types. As skin ages, or the barrier function degrades, it has more difficultly dealing with this type of stress.

Even when the skin re-adjusts to its more normal pH (4.5 to 5.5) - it is already damaged, irritated and stressed. The damage recovery involves longer term healing; 14 to 17 days for acid mantle repair. Continued long-term damage, stress, and mild irritation can prevent the skin from maintaining its best pH level of 4.5 to 5.5. With time, and increased damage, it may tend to stay at the 6 range or higher.

As skin become healthier, its pH values lower, and acne growth also lowers. The skin becomes more "normal" and regulated.

A product may include the term â€śpH Balancedâ€ť on the label. This does NOT mean that it has an optimal pH of 5.5. The term has no legal definition.

Some products are a good pH, but high in irritants. Which is better then high pH and high irritants, but not great either.

A good cleanser cleans the skin without breaking down the acid mantle, or adding irritants to the skin. It is mild with a 5.5 or lower pH.

Exfoliating Power of AHAs and BHAs

The acids commonly used in skin care treatment (AHAs, BHAs,TCA) are all pH dependent. The lower the pH, the more effective the acid is on the skin. It will work better, penetrate better and deeper, with a more concentrated effect. It can also be more irritating, because it is more â€śpowerful.â€ť

When an acid product is formulated with a pH below 2.0, all of its acid percentage is essentially â€śfreeâ€ť â€“ or available to work effectively on the skin.

For example:

A 20% lactic acid product at a pH of 1.9 has 20% of the lactic acid â€śfreeâ€ť or available to work on your skin. This is referred to as the â€śFree Acid Valueâ€ť or FAV.

As pH is raised closer to 7.0, less acid is free and the acid's effectiveness is weakened. So for a true understanding of the strength of your acid product, you need to know the acid percentage in the product, AND the pH.

AHAs (glycolic, lactic, mandelic, malic, citric, tartaric, etc.) are basically ineffective over 4.0 pH. To exfoliate the surface of the skin, the pH must be below 4.0. and the product percentage must be over 5%. 5% to 8% with a pH of 3.0 - 4.0 is the MINIMUM level for a daily use AHA; this will not treat scars or rebuild collagen.

BHAs (salicylic acid) are also basically ineffective over 4.0 pH. To exfoliate the skin, and unclog pores, the pH must be below 4.0 and the product percentage must be over 1%. (1% to 2% with a pH of 3.0 - 4.0 is the MINIMUM level for a daily use BHA. Skin with very clogged pores may need a higher concentration, up to 5%, and lower pH.

A pH test strip or litmus strip will give you the pH. (buy some made with a lower pH range, not all strips go from 1 â€“ 14) Strips will not be as accurate as a true electronic pH gage, but for home use it will be close enough.

Here is the link to Diana Yvonne's page on FAV and skin remodeling results:

For example: a 10% (free acid value) glycolic acid (AHA) gives "Significant increase in squamous cell turnover.Moderate increase in collagen deposition." In other words, 10% concentration of AHA acid working on the skin is enough to remove damaged cells, and rebuild skin collagen or firmness.

The second chart clarifies how the pH affects the amount of acid available for the skin to use (FAV).

For example, the three 20% entries:

20% AHA at a pH of 2.55 has a free acid value (FAV) of 18.6%. (It's only REALLY an 18.6% product in effectiveness, because of the pH over 2.0.)

20% at pH 3.4 drops to 14.6% FAV.

20% at pH 4.3 drops to 4.8% FAV.

When a 20% AHA product has a pH over 3 it has lost approximately 25% of its exfoliation activity. Once pH reaches 4.0, it loses 75% of its exfoliation activity. This is why so many commercial products are ineffective!

BHAs are effective at slightly different ranges, so the FAV scale differs a bit, but the principle is similar.

Skin Biology Exfol serum (2% BHA with a 3.2 pH) and Paulaâ€™s Choice 2% BHA (3.4 pH) are very similar products, and strengths. Professional Solutions 2% BHA has a pH of 1.9% so is actually much more active on the skin then either Exfol or Paulaâ€™s Choice, even at the same acid percentage.

Layering Products and Wait Times

pH affects the acid you use and how it works. Another consideration is not to disturb an acid while it is on your skin, which can alter pH and affect its action on the skin.

Generally speaking an acid is effective for approximately 20 to 30 minutes on the skin surface. After that point the skin pH has risen enough to lessen the exfoliation action on the skin, and the acid is effectively neutralized. During this 30 minute time frame, the acid is working at effective levels. Anything applied after 30 minutes will not hamper the effect of your acid, as the pH has already changed, and the acid action has slowed a lot.

Acids that are a high (peel) percentage such as 30% and 40% and above are more concentrated. Therefore their activity can continue for longer, so their action should be stopped at appropriate time with the use of a base rinse (baking soda and water) or a mild detergent cleanser. This will effectively halt the exfoliation action, by raising the skin pH.

Washing the skin with an cleanser of pH over 5.5 can slow or lessen the effect of your acid, because the acid must now deal with the high pH level on the skin. The bigger the range difference, the more this will affect your acid action, especially in the case of acids at 2.0 pH or less. So if you use a high pH cleanser, you should wait 30 minutes before using a pH-dependent product, to allow the skin to regulate its pH down to about 5.5. Or you can use an acidic toner to adjust the pH of the skin down before applying the product.

Most commercial astringents and toners contain high levels of irritants such as alcohols, Witch Hazel, Menthols, Mint oils, etc. These should be avoided, as they can damage the skin barrier. If you want to use a toner to adjust pH to lower levels, either use a liquid acid like Paulaâ€™s Choice BHA or AHA liquid first, or a very dilute mixture of Apple Cider Vinegar and distilled water. Vivant's Mandelic Acid toner is another good option, though it is expensive.

The best option is simply to use a cleanser with a pH of 5.5.

If you use a BHA product with 3.2 pH, then immediately apply a treatment product on top with 6.5 pH, the exfoliation action of your BHA is reduced by 75%. So applying a higher pH to your pH dependent acid will neutralize it sooner, or stop its action at the point you added the higher pH product.

Other products are also pH dependent. L-Ascorbic Acid forms of Vitamin C are pH dependent. For good stability and effective action, a good Ascorbic acid product is about 2.0 pH. Since Vitamin C absorbs faster into the skin, you can generally have a shorter wait of about 10 to 15 minutes.

Some Good General Rules

(This info comes from DebbieNIR, former host of the Diana Yvonne Skin Care Board. The original post is now gone.)

1. Use a mild, non-irritating, detergent-free cleanser with a 5.5 pH or lower.

2. If your cleanser pH is higher then 5.5, wait 15 to 30 minutes after cleansing to use a low pH AHA or BHA. (Or use a toner to acidify the skin first.)

3. Acids should be applied to freshly cleansed skin.

4. Both acid percentage and product pH must be used to factor total effective acid amounts. (FAV)

5. If using AHA and BHA together, always apply AHA first (after cleansing) â€“ UNLESS the BHA solution pH is equal to or lower than your AHA. In this case, apply the BHA first, because it penetrates skin oils better, and will enhance the AHA effectiveness.

6. Acids can be used in many different ways â€“ which affect wait times.

a) If a 2% or 5% BHA product is used, and rinsed off, WAIT until it is rinsed off to proceed with other products. (BHA washes are less effective than leave-on products.)

b) If a 2% or 5% BHA product is applied and left on the skin, but no AHA product is following, wait 30 minutes before applying next product. This allows the BHA to work at its most effective, and exfoliate at optimal levels.

c) If a 2% or 5% BHA is applied and left on the skin before a rinse-off AHA, you do not need to wait to apply the AHA â€“ IF the pH of both products is similar and compatible. If you rinse the AHA off after 5 minutes of application (over the BHA) you have also stopped the BHA action at 5 minutes. Both should have a lower effective pH. If the AHA has a higher pH, it will also raise the pH of the BHA, affecting its action also.

d) If a 2% or 5% BHA is being left on the skin, before a leave on AHA, you donâ€™t need to wait to apply the AHA. Apply BHA, then AHA, then wait a total of 30 minutes with both products layered, then proceed to next product application. (This is assuming that both acids have a low pH.)

e) Do not apply a higher pH BHA before a lower pH AHA. The higher pH product will hinder the lower pH product. Wait 30 minutes between both acids, applying the lowest pH first.

f) If using a higher BHA after a lower AHA product, apply lower pH AHA first. Wait 30 minutes for it to work. Then apply higher pH BHA on top, and wait 30 minutes for the BHA to work. Then apply other products on top.

g) If you need to rinse products off sooner then 30 minutes to shorten activity and lessen irritation, you can rinse with water, before proceeding to the next step. If using a high strength AHA or BHA, at strong â€śprofessionalâ€ť peel level strength, you will neutralize product with baking soda and water, and rinse off before proceeding to your next step.

h) Apply low pH before high pH.

Layering Other Products

This information is also from DebbieNIR's posts on the Diana Yvonne Skin Care Board.

Retinols, Retinoids, Vitamin A productsThese products are often pH dependent. Apply to clean dry skin, once to twice a day. Wait at least 15 to 30 minutes before adding other active products. In the case of retinols, retinyls, and retinals, wait at least 30 and preferably 45 minutes to allow the active ingredient to convert to retinoic acid within your skin.

DCX:This is an eye treatment product with soy. It is not an exfoliate. Should be applied to clean skin, once or twice a day. Apply after acids and cleansers, before copper and other treatments. Wait 10 minutes to allow it to absorb. Then apply other products.

DMAE:Can be used after acids and cleansing. Formulation type can dictate placement. Liquids, gels and serums can be applied before CP serums, lotions and creams. Lotion and cream DMAE should be applied after CP serums and creams. Wait 10 minutes to allow product to absorb.

Product Application by Type

This is a rough guide for product application, based on type/absorbency. This is for products that are not pH dependent, nor limited by activity type. The thickness of a product, dictates its placement. You cannot apply a thin product (like a liquid) over a thick product (like a cream) and expect it to absorb well. The cream can be a barrier to the thinner product.